As the internet and computer networking continue to evolve, data transmission speeds are increasing as well as the volume of data transmitted. The increase in data traffic is occurring in Local Area Networks (LANs) based on Ethernet and other transport mechanisms such as Wide Area Networks (WANs), and Storage Area Networks (SANs) which could use Ethernet or any of a number of data transport mechanisms. Similarly, the amount of data moving through Internet Protocol (IP) based networks such as the internet continues to grow substantially.
Accordingly, users face a growing need for new ways to store and maintain their data. Today's technology offers three basic storage options: Direct Attached Storage (DAS), Network Attached Storage (NAS) and Storage Area Networks (SAN).
In its most basic form, Direct Attached Storage consists of a disk drive directly attached to a personal computer or server. One of the most common methods of transferring data between a hard drive and its associated personal computer or server is the Small Computer Systems Interface (SCSI). Other methods, such as SATA and IDE are well known.
The SCSI protocol uses commands to transfer blocks of data, which are low level, granular units used by storage devices, as opposed to LANs, which typically use file based methods for transferring data. The overall operation and an architectural description of the SCSI protocol is available from the American National Standards Institute (ANSI), the specific specification having the designation ANSI/INCITS 366-2003, titled Information Technology—SCSI Architecture Model—2 (SAM-2), herein incorporated reference, and herein referred to as the SCSI Specification.
As internet traffic and storage needs have grown, there is a growing convergence between storage devices, protocols, and IP based transport mechanisms. For example, current SCSI storage devices are designed to work over a parallel cable having a maximum cable length of 12 meters, While IP based transport mechanisms have no data transmission distance limitation.
At the present time, the storage industry and the various industry entities responsible for developing and maintaining the various Internet Protocols are working together to develop standards to enable SCSI based data transfers over the internet. Specifically, the IP Storage (IPS) Working Group of the Internet Engineering Task Force (IEF) is in the process of finalizing a specification for encapsulating SCSI commands in the known TCP/IP protocol. The Internet SCSI (iSCSI) protocol for block storage is predicated on standard Ethernet transports. The iSCSI protocol defines the rules and processes to transmit and receive block storage data over TCP/IP networks, both of which employ error detection techniques to ensure data integrity during transmission.
iSCSI replaces the parallel SCSI direct cabling scheme with a network fabric. iSCSI is transport independent and will support any media that supports TCP/IP. Servers and storage devices that support iSCSI connect directly to an existing IP switch and router infrastructure. iSCSI enables SCSI-3 commands to be encapsulated in TCP packets and delivered reliability over IP networks. The iSCSI specification is complete and undergoing final ratification within the IETF. The current iSCSI specification is available from the IETF under the designation draft-ietf-ips-iscsi-20.txt, dated Jan. 19, 2003, and herein referred to as the iSCSI Specification. iSCSI network interfaces under development will be capable of transferring data over the internet in speeds approaching 20 Gbits/sec. The iSCSI protocol is just one example of a network storage protocol, which may employ the System and Method of the present invention, although those skilled in the art will appreciate that the System and Method of the present invention is useful in any type of data transfer protocol where CRC checksums are useful or required.
Cyclic Redundancy Check (CRC) techniques are used to verify the validity of data contained in data blocks or data segments transferred between devices, such as a storage device and a computer or server system, as described above. In other applications, CRC techniques may be used in data communication systems to verify the validity of data blocks transmitted between devices that are geographically distributed, such as those which employ the Internet as a transmission media, or in systems where data is transmitted over conventional telephone lines using modems.
A variety of CRC techniques are well known and have become widely deployed because of their highly reliable error detection capabilities, speed and relative ease of implementation. For example, a description of CRC applications may be found in the text: Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. “Cyclic Redundancy and Other Checksums.” Ch. 20.3 in Numerical Recipies in Fortran: the Art of Scientific Computing, 2nd ed., Cambridge, England: Cambridge University Press, pp. 888–895, 1992. A further example of a 32-bit CRC application is described in the industry standard publication designated: IEEE 802.3, 2000 Edition, CSMA/CD (ISO/IEC 8802-3:2000(E), which describes the requirements for what is commonly referred to as “Fast Ethernet”.
CRC values can be appended to a variety of data types ranging from blocks of data, having a predefined number of bits, to predefined groups of blocks, known as segments. CRC values are determined according to a predetermined algorithm which is known in both an initiator system and a target system. The actual CRC value is typically a function of the specific algorithm used, the actual data communicated, as well as the CRC value used in a prior communication session.
CRC algorithms can vary in complexity from those based on simple binary arithmetic to more complex algorithms using polynomials and some form of binary division or multiplication. The overall reliability of any given CRC algorithm is a function of the complexity of the algorithm used as well as the number of bits used in calculating the CRC value. For example, these algorithms may not detect an error for an N-bit checksum, because there is a possibility that ½N of random blocks will have the same checksum for inequivalent data blocks. However, as the value of N increases, the probability that two inequivalent blocks will have the same CRC value decreases. If N is sufficiently large, the probability of a detecting an error drops to a statistically insignificant value. Further, algorithms based on polynomial division or multiplication are less susceptible to error than those based on simple addition or subtraction.
In general, CRC values are generated using binary or Modulo-2 arithmetic. Therefore, the multiplication process used to generate a CRC value in a complex polynomial is merely a sequence of logical ANDs and XORs which can be readily implemented with well known hardware or software techniques. To detect errors in any given data communication, a CRC value is calculated prior to data transmission and appended to a data packet or segment. When the data packet or segment is received, the CRC value is recalculated, and compared to the CRC value appended to the data packet or segment. If the two CRC values match, no errors occurred during data transmission. If the two CRC values do not match, an error has occurred, and re-transmission is required.